handle GitHub issues without a milestone

The previous version only handled GitHub issues
 - either with a milestone having a deadline (try clause)
 - or with a milestione without a deadline (except KeyError clause)

This commit handles the case when there's no milestone set at all, and a
TypeError is raised upon subscripting the invalid issue['milestone']
with the value 'due_on'.

github_issues.py

@@ -49,7 +49,7 @@ def issue2entry(issue):
 	url = issue['html_url']
 	try:
 		due = issue['milestone']['due_on']
-	except KeyError:
+	except (KeyError, TypeError):
 		due = issue['created_at']
 	return {
 			'subtitle': HTML_RE.search(url).group(1),